This invention relates to excavating and similar type machines, which have bodies carrying loadsupporting booms, or projections, extending at one end, the bodies being mounted for rotation upon a base, and more particularly to the mounting which supports, and permits rotation of, the body on the base.
Excavating machines, either draglines or shovels, have houses enclosing the operating machinery, with the houses being supported upon bases by means of roller circles serving as anti-friction bearings as the houses rotate about central, vertical pivot shafts carried by the bases. Due to the fact that long, inclined booms are attached at their bottoms to the fronts of the houses, and excavating buckets and their loads are suspended from the booms at some distance forward of the houses, the loads imposed upon the roller circles are offset from the center of house rotation, with the points of greatest load shifting about the roller circles as the houses are rotated. The resulting uneven, constantly shifting, weight distribution about the circle has presented many problems in roller circle design and operation.
The problem has increased enormously as the size and weight of the machines have grown over the years. With the requirement for machines having tremendous bucket capacity, resulting in machines weighing millions of pounds, the problem has become quite critical.
In order to solve the problem, a live roller circle has been used. It consists of a circular rail attached to the underside of the upper frame, resting on a complete circle of rollers (usually forty or more, guided and/or separated by a cage) which in turn rests upon a circular rail mounted upon the top of a lower frame, or tub in the case of a dragline. It is called "live" in order to distinguish it from other arrangements in common use in which rollers are mounted on shafts fixed to the upper frame. Ordinarily, it is made to withstand only downward loads, the other possible loads (radial and upward) being taken by a journal and a thrust face at the center of rotation. A live roller circle thus resembles a very large roller thrust bearing, but with the following differences in detail and application. First, the rails for the live roller circle usually are made in segments and some segments are usually left out of the upper rail at the sides of the upper frame; and, second, the centroid of the load does not remain at, or near, the center of rotation, as in an ordinary thrust bearing, but moves forward or rearward a considerable distance, even going out beyond the edge of the circle in extreme instances.
In order to design a live roller circle for adequate life, it is necessary to compute loads which individual rollers must withstand. In such computations, it is customary to assume that the upper and lower frames are uniformly elastic in the load-carrying regions above and below the roller circle. Thus, if the centroid of the load is ahead of center, the roller or rollers at the front of the circle would be most heavily loaded, and roller loads would diminish toward the rear. In other words, the circle as a whole would be treated as a beam or column section under an offset compressive load.
For many years it has been known that the actual load distribution is quite different from the ideal, and that maximum individual roller loads are often as much as twice those which would be computed by the classical beam theory. There are two main reasons for this. First, the frames are not uniformly elastic, and loads tend to be concentrated on parts of the roller circle which are located directly below or above areas where heavy loads are applied to the frames, such as at the foot of the boom; and, second, the rail paths, or surfaces, to which the rails are attached, are not initially flat, since it is very difficult to build structures as large as these frames with the relative precision usually associated with anti-friction bearings.
To improve roller load distribution, efforts in recent years have taken the following approaches:
1. Make the frames deeper, thus stiffer against gross bending deflection and better able to spread out loads around the circle. A deeper frame also has greater "local" softness over the circle, because it has greater depth of web which is subjected to direct compressive load.
2. Apply loads to the upper frame at more logical locations, with relation to the roller circle. This has been done to some extent by relocating boom foot mountings, gantry and mast locations.
3. Machine the rail paths at the erection site, after the frames are completely assembled and welded, using optical instruments to check flatness.
None of these has completely solved the problem, and the field machining is a very expensive, time-consuming operation.
Because of the inability to achieve true flatness in the circle rail, or in the taper angle of tapered rollers if used, it has been necessary to use fully crowned rollers to prevent all of the roller load being imposed near one end of the roller, resulting in destructively high contact stress. Even without geometric errors, the occasional instance of the centroid moving to the edge of the circle and the resulting take-up of clearance on the thrust fact at the center journal will cause enough tipping movement to chase the roller load to the outer end. Fully crowned rollers, when loaded, have a contact area against a flat rail which is elliptical, with the greatest width and highest shear being developed only at the center. This is in distinction to a straight roller with crowned ends which will carry more load at the same stress level, because the contact area has an oval shape with stress being uniform over a considerable length.
Another problem encountered is the tendency for rail segments to creep against the frame and wear into it. This occurs principally because the frame surface undergoes strains, or elastic changes of length, as the machine works. The rail segments, being long and heavy, and not completely integral with the frame, do not change length very much. Therefore, one end or the other of a segment will move very slightly with respect to the frame. Then, when the roller load happens to be greater on that end, the frame strain may suddenly be relieved, causing the other end of the segment to move.